Phase noise in radar systems can inhibit target detection due to instabilities that affect a Moving Target Indicator (MTI) function of the radar. Instabilities in the transmitted pulse, RF oscillators, receiver amplifiers and RF signal paths in a radar system can all be sources of phase noise. The total phase noise of a radar system can cause targets to be lost against background clutter returns. This is becoming more critical in military surveillance radar systems due to stealth technology and the increased use of small unmanned aircraft.
Surveillance radar systems use MTI processing to discern targets against clutter returns; however, excessive phase noise can impact clutter cancellation. The phase noise impact on clutter cancellation causes targets to fall below the threshold of detection of the Constant False Alarm Rate (CFAR) of the radar system. This unfortunately results in targets that are invisible to the surveillance system.
It is difficult to measure and quantify the phase noise present in a fully installed and integrated system on-board an aircraft or ship, or other vehicle or vessel. Varying power levels, frequencies and long signal paths make measurement of phase noise using conventional techniques quite challenging. Phase noise measurements are usually confined to individual components prior to the assembly of the radar system. Factory specification of radar subassemblies is intended to ensure that the radar system will have the desired target sensitivity and clutter rejection. It is unlikely, however, that once a component is installed in the radar system that any degradation in phase stability will be detected by ordinary system operation. Over time, a radar operator may notice some difficulty in target detection, but unless a means of quantifying the total system phase noise is employed, the degradation will often go unchecked and the surveillance system could be compromised.
Specification of the allowable phase noise in components for use in the RF oscillator and amplifier sections establish a phase noise floor and target detection sensitivity for the radar system. However, once a radar system is installed and placed into regular service, for example, on-board an aircraft, ship or other vehicle or vessel, the radar system phase noise is not quantified, nor is the impact of phase noise on target detection accurately measured.